How to solve 'Header has been written. Want to overwrite status code 200 with 400' in Gin

php editor Zimo introduces you to the method to solve the "Header has been written. Want to overwrite status code 200 with 400" problem in Gin. This problem usually occurs when using PHP for web development, and may cause the web page to fail to load properly. Solutions include checking for duplicate output in your code, clearing buffers, checking server configuration, etc. By taking the appropriate steps, you can easily resolve this issue and ensure that your pages function properly.

Question content

I'm trying to use the Gin framework to calculate a report independently of the HTTP request, but I also want to return it to the request if it hasn't been closed yet. The following code snippet sometimes works fine but sometimes throws the error "Header already written. Wanted to overwrite status code 200 with 400". I can't find a solution.

// router.go
reportRoute.POST("/query", func(c *gin.Context) {
        reporter.ReportInChan <- c
    }) // Unpublish report by id

//reporter
package reporter

import (
    "fmt"
    "github.com/gin-gonic/gin"
)

var (
    ReportInChan = make(chan *gin.Context)
)

func Listener() {
    for {
        select {
        case req := <-ReportInChan:
            queryReports(req)
        }
    }
}

func queryReports(req *gin.Context) {
    type ReqQuery struct {
        QueryID    uint           `json:"query_id"`
        Parameters map[string]any `json:"parameters"`
    }
    type Req struct {
        ReportID uint       `json:"report_id"`
        Queries  []ReqQuery `json:"queries"`
    }

    var reqBody Req
    err := req.BindJSON(&reqBody)
    if err != nil {
        fmt.Println("hata var")
        if ctx := req.Request.Context(); ctx.Done() == nil {
            req.JSON(400, gin.H{"error": "Veriler Yanlış Gönderiliyor. Lütfen Bilgi İşlem Birimiyle İletişime Geçin ", "data": nil})
        }
        return
    }
}

Workaround

The handler below will send the context to the channel and then return. Once the handler returns, the request will be canceled. At this point, the receiving goroutine may have no chance of producing any output at all, but sometimes it may produce any output. If the receiving Goroutine starts producing output after the Gin framework writes the response but before the context is canceled, you will receive the above error.

reportRoute.POST("/query", func(c *gin.Context) {
        reporter.ReportInChan <- c
})

What you really need to do is implement a timeout scheme, so if the request handler can respond before that timeout, it is written to the output and returned. Otherwise, you let the request handler run and find another way to return the result to the caller, perhaps through another API call:

type reportRequest struct {
    ReportParams
    result chan ReportResult
}

reportRoute.POST("/query", func(c *gin.Context) {
        req:=reportRequest {
              ReportParams: {...},
              result: make(chan ReportResult),
        }
        reporter.ReportInChan <- req
        select {
           case time.After(100*time.Millisecond): 
               // Wait 100 msecs 
           case result<-req.result:
               // Write the result
        }
})

The above code will create a report request from the request context, create a return channel, and then send it to the report processor. If the report processor responds within 100 milliseconds, the results can be written. Otherwise, the results should be stored elsewhere where they can be retrieved by another API.

The report processor should look like this:

func (r Reporter) reportProcessor() {
   for request:=range r.ReportInChan {
       result:=processReport(request)
       select {
          case request.result <- result:
              // Wrote the result
          default:
               // Cannot write the result, handler returned
             storeResult(result)
   }
}

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